Modified Resin Systems for Liquid Resin Infusion Applications &amp; Process Methods Related Thereto

ABSTRACT

Embodiments of the invention are directed to modified resin systems for use in liquid resin infusion (LRI) processes, variations of LRI processes and other suitable processes. In one embodiment, the modified resin system includes a novel combination of at least one base resin, an amount of particles within a predetermined range and an amount of thermoplastic material within a predetermined range wherein, when combined, the modified resin system has an average viscosity below a threshold average viscosity at a specific temperature and a high level of toughness. The modified resin system may additionally include a curing agent and other suitable components. The modified resin system has been experimentally shown to exhibit a unique, controllable and constant morphology which may be at least partially responsible for imparting a required toughness and damage resistance to a finished composite without adversely impacting properties such as viscosity, potlife, cure temperature, glass transition temperature or tensile modulus of the modified resin system.

FIELD OF INVENTION

Modified resin systems for liquid resin infusion applications, prepregautoclave applications and hybrids thereof.

BACKGROUND OF INVENTION

Liquid resin infusion (LRI) is a process used to manufacturefiber-reinforced composite structures and components for use in a rangeof different industries including the aerospace, transport, electronics,and building and leisure industries. The general concept in LRItechnology involves infusing resins into a fiber reinforcement, fabricor a pre-shaped fibrous reinforcement (“preform”) by placing thematerial or preform into a mold (two-component mold or single-sidedmold) and then injecting resin under high pressure (or ambient pressure)into the mold cavity or vacuum bag sealed single-sided mold. The resininfuses into the material or preform resulting in a fiber-reinforcedcomposite structure. LRI technology is especially useful inmanufacturing complex-shaped structures which are otherwise difficult tomanufacture using conventional technologies. Variation of liquid resininfusion processes include, but are not limited to, Resin Infusion withFlexible Tooling (RIFT), Constant Pressure Infusion (CPI), Bulk ResinInfusion (BRI), Controlled Atmospheric Pressure Resin Infusion (CAPRI),Resin Transfer Molding (RTM), Seemann Composites Resin Infusion MoldingProcess (SCRIMP), Vacuum-assisted Resin Infusion (VARA) andVacuum-assisted Resin Transfer Molding (VARTM).

Most resin infusion systems are inherently brittle, and the viscositylevels necessary to achieve the injection process preclude the use oftoughening agents. Said differently, the properties of toughness and lowviscosity are typically mutually exclusive in conventional resininfusion systems. In prepregs, high levels of toughness are generallyachieved through the addition of about ten percent (10%) to about thirtypercent (30%) by weight of a thermoplastic toughener to the base resin.However, addition of such tougheners to LRI systems generally results inan unacceptable increase in the viscosity of the resin and/or reductionin resistance of the cured material to solvents. In the specific case ofparticulate toughener, there may be additional filtering issues in thetextile. These limitations render the addition of toughenersconventionally added in prepregs generally unsuitable in conventionalLRI applications.

One technology to toughen fiber-reinforced composite structuresmanufactured by LRI technologies is to integrate the toughener into thepreform itself. For example, a soluble toughening fiber may be directlywoven into the preform thereby eliminating the need to add toughenerinto the resin which otherwise would increase the viscosity of the resin(rendering it unsuitable for resin infusion). Another example is the useof soluble or insoluble veils comprising of toughener used as aninterleaf with the reinforcement of the preform. However, in either ofthese methods, the manufacturing process may be more complicated andcostly, in addition to increasing the risk of hot/wet performanceknock-downs and solvent sensitivity with a polymer based insolubleinterleaf. Another technology is the addition of particles to the resin.The amount of particles required to reach a suitable toughnessthreshold, however, is often high resulting in a viscous resin requiringa very narrow process window that is generally unfavorable for LRI.

SUMMARY OF INVENTION

A formulation, comprising: (i) at least one base resin; (ii) an amountof particles within a predetermined range in a carrier resin; and (iii)an amount of thermoplastic material within a predetermined range whereinthe base resin, the particles and the thermoplastic material arecombined to form a modified resin system, the modified resin having anaverage viscosity below a threshold average viscosity within apredetermined temperature range is herein disclosed. The formulation mayfurther comprise a curing agent. The curing agent may be ananiline-based amine compound. The base resin may be one of epoxy,bismaleimide, cyanate ester or a combination thereof. The base resin maybe a combination of epoxies including at least one di-, tri- ortetra-epoxy. The particles may be one of chemically functionalized orchemically non-functionalized core-shell rubber particles or hollowparticles. A material comprising the core may be one ofpolybutadiene-styrene, polybutadiene or a combination thereof, and amaterial comprising the shell may be one of silica, polymerized monomersof acrylic acid derivatives containing the acryl group including acrylicand poly(methyl methacrylate) or a combination thereof. In a curedcondition, the particles may be substantially uniformly dispersedthroughout the modified resin system. The thermoplastic material may beone of phenoxy-based polymers, poly(ether sulfone) polymers, poly(etherether sulfones), poly(methyl methacrylate) polymers, carboxylterminatedbutadiene acrylonitrile polymers, copolymers thereof, or combinationsthereof. The formulation wherein the amount of thermoplastic material isbelow approximately 30% net weight, preferably below 7%, of the modifiedresin system. In a cured condition, at least the thermoplastic materialphase may separate from the base resin. More particularly, thethermoplastic material phase may separate into aggregate domains fromthe base resin, each aggregate domain having an island-like morphology.The morphology in a cured article may evolve: (i) during the laterstages of a ramp to dwell temperature; or (ii) after a ramp to dwell hasbeen completed during the cure cycle. The amount of particles and theamount of thermoplastic material may be combined in a 1 to 0.56 ratio.The threshold average viscosity may be less than 5 Poise at atemperature of less than 180° C., more narrowly between 80° C. and 130°C.

A composite article, comprising: a structure having a predeterminedshape, the structure having a plurality of layers of a fiber-basedfabric, the structure having a targeted composite toughness within apredetermined range, wherein the toughness is at least partiallyimparted by a modified resin system during a process, the modified resinsystem including: (i) at least one base resin; (ii) an amount ofparticles within a predetermined range in a carrier resin; and (iii) anamount of thermoplastic material within a predetermined range whereinthe base resin, the particles and the thermoplastic material arecombined to form the modified resin system, the modified resin having aaverage viscosity below a threshold average viscosity within apredetermined temperature range is herein disclosed.

The modified resin system may further include a curing agent, the curingagent comprising an aniline-based amine compound. The base resin may beone of epoxy, bismaleimide, cyanate ester or a combination thereof. Thebase resin may include a combination of epoxies including at least onedi-, tri- or tetra-epoxy. The particles may be one of core-shell rubber(CSR) particles or hollow particles wherein, when the particles are CSRparticles, a material comprising the core is one ofpolybutadiene-styrene, polybutadiene or a combination thereof, and amaterial comprising the shell is one of silica, polymerized monomers ofacrylic acid derivatives containing the acryl group including acrylicand poly(methyl methacrylate) or a combination thereof. In a curedcondition, the particles may be substantially uniformly dispersedthroughout the modified resin system. The thermoplastic material may beone of phenoxy-based polymers, poly(ether sulfone) polymers, poly(etherether sulfones), polymerized monomers of acrylic acid derivativescontaining the acryl group including acrylic and poly(methylmethacrylate) polymers, carboxylterminated butadiene acrylonitrilepolymers, copolymers thereof, or combinations thereof. The amount ofthermoplastic material is below approximately 30% net weight, preferablybelow 7% net weight, of the modified resin system. With the base resinin a partially cured or gel-like state, the thermoplastic material mayseparate into aggregate domains from the base resin, each aggregatedomain having an island-like morphology. The amount of particles and theamount of thermoplastic material may be combined in a 1 to 0.56 ratio.The structure may exhibit a high level of microcrack resistance. Thethreshold average viscosity may be less than 5 Poise at a temperature ofless than 180° C., more narrowly between 80° C. to 130° C. Thefiber-based fabric may be comprised of reinforcing fibers of a materialselected from the group consisting of organic polymer, inorganicpolymer, carbon, glass, inorganic oxide, carbide, ceramic, metal or acombination thereof. The process may be a liquid resin infusionmanufacturing process, a prepreg manufacturing process or a resin filminfusion process.

A formulation, comprising: (i) a base resin comprising at least oneepoxy; (ii) a curing agent; (iii) an amount of thermoplastic material;and (iv) an amount of core-shell particles wherein the base resin, thecuring agent, the thermoplastic material and the particles are combinedto form the modified resin system, the modified resin having an amountof thermoplastic material of less 30% net weight, preferably less than7% net weight, of the total weight of the modified resin system isherein disclosed.

With the base resin in a partially cured or gel-like state, thethermoplastic material phase may separate into aggregate domains fromthe base resin. The amount of particles and the amount of thermoplasticmaterial may be combined in a 1 to 0.56 ratio. With the base resin in apartially cured, gel-like, cured or vitrified state the particles aresubstantially uniformly dispersed throughout the modified resin system.The modified resin system may have an average viscosity of less than 5Poise at a temperature of less than 180° C., more narrowly between 80°C. and 130° C. With the base resin in a cured or vitrified condition,the thermoplastic material may separate into aggregate domains from thebase resin, each aggregate domain having an island-like morphology. Themorphology in a cured article may evolve (i) during the later stages ofa ramp to dwell temperature or (ii) after a ramp to dwell has beencompleted during the cure cycle.

A manufacturing process, comprising: (i) preparing a preform; (ii)laying the preform within a mold; (iii) heating the mold to apredetermined temperature; and (iv) injecting a resin wherein the resinis a modified resin, the modified resin system comprising a combinationof (i) at least one base resin; (ii) a curing agent; (iii) an amount ofparticles within a predetermined range in a carrier resin; and (iv) anamount of thermoplastic material within a predetermined range whereinthe amount of thermoplastic material of the modified resin is less than30% net weight, preferably less than 7% net weight, of the total weightof the modified resin system is herein disclosed.

The predetermined temperature of the mold may be 110° C. Themanufacturing process may further comprise ramping a temperature of themold to 180° C. at a rate of less than 10° C. per minute, more narrowly,less than 5° C. per minute. The manufacturing process wherein, when themold reaches 180° C., the temperature is held for between 90 minutes and150 minutes. The preform may be sealed within the mold by at least avacuum bag. An average viscosity of the modified resin system may beless than 5 Poise at a temperature range of less than 180° C., morenarrowly between 80° C. and 130° C. The preform may be comprised ofplurality of layers of fiber-based fabric. The fiber-based fabric mayhave a structure comprising one of woven fabrics, multi-warp knittedfabrics, non-crimp fabrics, unidirectional fabrics, braided socks andfabrics, narrow fabrics and tapes or fully-fashioned knit fabrics. Thefiber-based fabric may be comprised of reinforcing fibers of a materialsuch as organic polymer, inorganic polymer, carbon, glass, inorganicoxide, carbide, ceramic, metal or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating conventional toughened resin systemsand the modified resin system according to an embodiment of theinvention.

FIG. 2 is a chart showing the relationship between viscosity andtoughness for a thermoplastic material in a base resin, core-shellparticles a base resin, and a combination of thermoplastic material andcore-shell particles in a base resin according to an embodiment of theinvention.

FIG. 3A is an optical micrograph of thermally-evolved thermoplasticdomains and core-shell rubber particle regions at increasingconcentration, but at a constant ratio of core-shellparticles:thermoplastic toughener in a modified resin system accordingto an embodiment of the invention.

FIG. 3B is an optical microscopy evaluation of the thermally evolvedthermoplastic domains and CSR particle regions in the proposed inventiondemonstrating the influence of CSR concentration on the dimensions ofthe witnessed morphology.

FIG. 4 is a scanning electron microscopy (SEM) image of the island likemorphology and core shell particles witnessed in a cured and modifiedresin system with respect to damage resistance mechanisms according toan embodiment of the invention.

FIG. 5 is a graph comparing the fracture toughness of a modified resinsystem according to embodiments of the invention to the fracturetoughness of other resin systems.

FIG. 6 Describes the evolution of the morphology as represented by anembodiment of the current invention as a function of temperature orvitrfication onset in the base resin comprising the proposed invention.

FIGS. 7A and 7B are Transmission electron Microscopy images of theisland like morphology and core shell particles witnessed in a cured andmodified resin system with respect to damage resistance mechanismsaccording to an embodiment of the invention.

FIG. 8 Is an expanded SEM image detailing the growth rings in thethermoplastic domains present in the proposed invention.

FIG. 9 illustrates a schematic of the generalized morphology of amodified resin system according to embodiments of the invention

FIG. 10 Illustrates a representative LRI system having a fabric performthereon.

FIG. 11 is a chart comparing CSAI values of the modified resin systemaccording to embodiments of the invention to the CSAI values for otherresin systems.

DETAILED DESCRIPTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention.

Embodiments of the invention are directed to modified resin systems foruse in resin infusion (RI) processes, variations of LRI processes andother suitable processes such as prepreg processes. In one embodiment,the modified resin system includes a novel combination of at least onebase resin, an amount of particles within a predetermined range and anamount of thermoplastic material within a predetermined range wherein,when combined, the modified resin system has an average viscosity belowa threshold average viscosity within a specific temperature range and ahigh level of toughness. The modified resin system may additionallyinclude a curing agent and other suitable components. The modified resinsystem has been experimentally shown to exhibit a unique, controllableand constant morphology which is substantially or completely responsiblefor imparting a required toughness and damage resistance to a finishedcomposite article without adversely impacting resin properties such asviscosity, potlife, cure temperature, glass transition temperature ortensile modulus of the modified resin system.

According to embodiments of the invention, a combination of at least onebase resin, an amount of particles within a predetermined range and anamount of thermoplastic material within a predetermined range, inaddition to other components, may be combined in a “one pot” formulationto generate a modified resin system which can be used in RI/LRIprocesses or prepreg processes. The modified resin system as formulatedaccording to embodiments of the invention was discovered to have anunexpectedly low viscosity, low reactivity, a high level of toughness(G_(1C)), among other characteristics, when subjected to numerousexperimental tests. It is anticipated that the modified resin may alsobe used in variations of liquid resin infusion processes including, butnot limited to, Resin Infusion with Flexible Tooling (RIFT), ConstantPressure Infusion (CPI), Bulk Resin Infusion (BRI), ControlledAtmospheric Pressure Resin Infusion (CAPRI), Resin Transfer Molding(RTM), Seemann Composites Resin infusion Molding Process (SCRIMP),Vacuum-assisted Resin Infusion (VARI), Resin Transfer Injection (RTI)and Vacuum-assisted Resin Transfer Molding (VARTM) as well as otherprocesses used to manufacture composite articles.

FIG. 1 is a schematic illustrating conventional resin systems and themodified resin system according to an embodiment of the invention.Numerical reference 102 represents an unmodified neat epoxy which may beused in composite manufacturing processes. An unmodified epoxy resinsystem is generally known to be unsuitable in the manufacture of hightoughness composite articles without resorting to the use of asecondary, insoluble toughening article such as a hot-melt adhesive web,e.g., SPUNFAB® veil. Numerical reference 104 represents a modified epoxysystem having core-shell rubber (CSR) particles therein to impart atoughening characteristic. Typically, modified epoxy systems of thistype are known to exhibit high toughness values which often do nottranslate into equivalent composite performance. Numerical reference 106represents another modified epoxy system having a thermoplastic therein.This modified epoxy system is known to have an average viscosity whichis outside of acceptable processing windows for LRI applications.

Numerical reference 108 represents a modified resin system according toembodiments of the invention which is characterized by having a suitableaverage viscosity for LRI (e.g., less than 5 Poise) without sacrificingperformance in the resin or composite, specifically related to toughnessproperties. Modified resin system 108 includes at least one base resin,an amount of particles within a predetermined range and an amount ofthermoplastic material within a predetermined range in a novelcombination which makes it suitable for LRI processes, prepreg processesand other like processes. In FIG. 1, the base resin is an epoxy resin orcombination of epoxy resins; however, embodiments of the invention arenot limited to epoxy resins.

In the context of this application, a “resin” is a synthetic polymercompound which begins in a viscous state and hardens with treatment.Resins are used as a structural matrix material in the manufacture ofadhesives and composites and are often reinforced with fibers (e.g.,glass, Kevlar, Boron and Carbon). In some embodiments, the base resinmay be any one of epoxy, bismaleimide, benzoxazine, cyanate ester, vinylester, polyisocyanurates, bismalimide, cyanate ester, phenolic resin orany combination thereof in addition to other suitable resins. In someembodiments, the base resin is an epoxy resin or a combination of epoxyresins. The epoxy resin may be a tetra-, tri-, di-epoxy or combinationsof tetra-, tri- and/or di-epoxies. Exemplary tri-epoxies includetriglycidyl p-aminophenol (MY-0510 available from Huntsman AdvancedMaterials, Inc.) and ARALDITE® (MY-0600 available from Huntsman AdvancedMaterials, Inc.). An exemplary tetra-epoxy is tetraglycidyldiaminodiphenyl methane (MY-721 available from Huntsman AdvancedMaterials, Inc.). Other suitable epoxy resins include bisphenol F epoxy(PY-306 available from Ciba Geigy).

In the context of this application, a “particle” is a polymer-basedmaterial having a core-shell or hollow morphology. Core-shell rubber(CSR) particles have the characteristic of having a core comprising of arubbery material surrounded by an outer shell of glassy material. CSRparticles are used as toughening agents when combined with polymericmatrices, e.g., epoxy resins. In some embodiments, the particles may beany commercially available chemically functionalized or chemically nonfunctionalized CSR particles having a core material ofpolybutadiene-styrene or polybutadiene and having a shell material ofsilica or polymerized monomers of acrylic acid derivatives containingthe acryl group including acrylic and poly(methyl methacrylate). The CSRparticles may be supplied in a carrier resin such as tetraglycidyldiaminodiphenyl methane (i.e., MY-721) and may have a diameter ofbetween about fifty (50) nanometers (nm) and about eight hundred (800)nm, in one embodiment, about one-hundred (100) nm. Examples ofcommercially available CSR particles include, but are not limited to,the Paraloid series of materials (available from Rohm and Haas), MX411(polybutadiene-styrene/acrylic) and MX416 (polybutadiene/acrylic) (bothare dispersions in Huntsman MY721 epoxy resin and are available fromKaneka Corp.); however, any particle exhibiting the CSR or hollowstructure as described above may be used in the modified resin systemsaccording to embodiments of the invention.

Core-shell particles have been evidenced to toughen LRI systems via acavitation mechanism in addition to crack pinning or “tear out”mechanisms. In a cavitation mechanism, the rubbery cores of the CSRparticles yield under the stress concentrations at a crack tip,resulting in dissipation of energy from the crack front and theformation of voids in the core material.

In the context of this application, a “thermoplastic” is a polymer thatis elastic and flexible above a glass transition temperature (T_(g)). Insome embodiments, the thermoplastic Material comprises one ofphenoxy-based polymers, poly(ether sulfone) (PES) polymers, poly(etherether sulfones), polymerized monomers of acrylic acid derivativescontaining the acryl group including acrylic and poly(methylmethacrylate) (PMMA) polymers, carboxyl terminated butadieneacrylonitrile (CTBN) polymers, copolymers thereof; or combinationsthereof. Representative thermoplastics include, but are not limited to,KM180 (available from Cytec Industries. Inc.), 5003P (available fromSumitomo Corp.), PKHB (InChemRes); however, any thermoplastic or othersuitable material (e.g., Nanostrength X, available from Arkema, Inc.)exhibiting a thermally driven phase separation from a base resin, moreparticularly, exhibiting aggregate domains, or an “island-like”morphology (explained in more detail below), may be used in the modifiedresin systems according to embodiments of the invention.

An example of a typical mechanism for thermoplastic toughening ofcomposite or resin matrices is crack pinning. Indications of crackpinning mechanisms include tailing behind thermoplastic domains orapparent plastic deformation around such thermoplastic zones originatingfrom a divergent crack front around a thermoplastic rich region andsubsequent convergence of the split crack fronts. Another example of atypical toughening mechanism is that of ductile tearing which can bedescribed as a localized plastic deformation upon application of astress to the material.

A “curing agent” is a substance or mixture of substances added to apolymer composition (e.g., resin) to promote or control the curingreaction. Addition of curing agent functions to toughen and harden apolymer material by cross-linking of polymer chains. Representativecuring agents include, but are not limited to, methylenebis(3-chloro-2,6 diethylaniline) (MCDEA), 3,3′-diaminodiphenyl sulfone(3,3′-DDS), 4,4′-diaminodiphenyl sulfone (4,4′-DDS), dicyandiamide(DICY), N-methyl-diethanolamine (MDEA) and4,4′-methylene-bis-(2-isopropyl-6-methyl-aniline) (MMIPA).

According to embodiments of the invention, the modified resin system mayinclude a thermoplastic which is 7% or less net weight of the modifiedresin system combined with an amount of CSR particles in a 1 to 0.56ratio of thermoplastic to CSR particles. In one embodiment, the baseresin may be a combination of di-, tetra- and tri-epoxies such asPY-306, MY-0500 and/or MY-0600). In one embodiment, the thermoplasticmaterial may be 5003P and the CSR particles may be MX411 (in MY-721) orMX416 (in MY-721) one-hundred (100) nm particles. A curing agent, suchas MCDEA may be added to the “one pot” resin system to make the resinsystem curable when heat and/or pressure is/are applied thereto.

The formulation of the present invention comprises at least one baseresin; an amount of particles within a predetermined range in a carrierresin; and an amount of thermoplastic material within a predeterminedrange wherein the base resin, the particles and the thermoplasticmaterial are combined to form a modified resin system, the modifiedresin having an average viscosity below a threshold average viscositywithin a predetermined temperature range. The threshold averageviscosity of the formulation is less than 5 Poise at a temperature ofless than 180° C. and preferably at a temperature of between 80° C. and130° C.

When the formulation is in a cured condition, at least the thermoplasticmaterial is phase separated from the base resin and preferably phaseseparates into aggregate domains from the base resin, each aggregatedomain having an island-like morphology. The cure morphology evolves (i)during the later stages of a ramp to dwell temperature or (ii) after aramp to dwell has been completed during the cure cycle.

The amount of thermoplastic material in the formulation is belowapproximately 30% net weight of the modified resin system and preferablybelow approximately 7% net weight of the modified resin system.

The formulation may include an amount of particles and the amount ofthermoplastic material combined in a 1 to 0.56 ratio.

When the formulation is in a cured condition, the thermoplastic materialis phase separated from the base resin and preferably, the thermoplasticmaterial phase separates into aggregate domains from the base resin,each aggregate domain having an island-like morphology.

Further embodiments of the present invention include a manufacturingprocess, comprising preparing a preform, laying the preform within amold, heating the mold to a predetermined temperature and injecting aresin wherein the resin is a modified resin, the modified resin systemcomprising a combination of: (i) at least one base resin; (ii) a curingagent; (iii) an amount of particles within a predetermined range in acarrier resin; and (iv) an amount of thermoplastic material within apredetermined range wherein the amount of thermoplastic material of themodified resin is less than 30% net weight of the total weight of themodified resin system.

The above manufacturing process may further modified wherein thepredetermined temperature of the mold is between 90° C. and 120° C. ormore preferably the predetermined temperature of the mold is 110° C.

The manufacturing process may be practiced by ramping a temperature ofthe mold to 180° C. at a rate of up to 5° C. per minute or morepreferably at a rate of 2° C. per minute.

Furthermore, when the mold reaches 180° C., the temperature may be heldabout 120 minutes.

The manufacturing process may be practiced wherein the preform is aplurality of layers of fiber-based fabric. The fiber-based fabric mayhave a structure comprising one of woven fabrics, multi-warp knittedfabrics, non-crimp fabrics, unidirectional fabrics, braided socks andfabrics, narrow fabrics and tapes or fully-fashioned knit fabrics. Thefiber-based fabric may utilize reinforcing fibers of a material selectedfrom the group consisting of organic polymer, inorganic polymer, carbon,glass, inorganic oxide, carbide, ceramic, metal or a combinationthereof.

Furthermore, the manufacturing process is preferably practiced where thepreform is sealed within the mold by at least a vacuum bag.

Representative formulations according to embodiments of the inventionwere prepared according to the following general Example:

Example 1

A base resin having di-, tri- and tetra-epoxies, a quantity of aminecuring agent and quantities of 5003P thermoplastic and CSR particles(i.e. MX411) were combined. The combination (100 grams) was transferredinto steel molds which were then placed in a fan oven preheated to 100°C. (ramp to 180° C. at 1° C. per minute, dwell for 2 hrs ramp to 25° C.at 2° C. per minute). Samples (prepared from the cured modified resinplaque) were prepared according to the relevant ASTM standard for thedesired test.

Example A Effect of Thermoplastic and CSR Concentrations on ResinToughness

Experiments were conducted to quantify the effect of thermoplastic(i.e., 5003P) in the absence of core-shell particles (i.e. MX411) (andvice versa, i.e., core-shell particles) as toughening agents, therebyproviding a baseline for the toughening mechanism in the formulationaccording to embodiments of the invention. The viscosity (η) in the baseresin system (containing no CSR particles) was observed to increase asthe percentage loading of thermoplastic was increased, but to beindependent of CSR concentration. The toughness (G_(1C)) of the systemswas found to increase with both increasing thermoplastic and CSRconcentration. It can be appreciated by one of ordinary skill in the artthat the use of CSR particles to achieve a high resin G_(1C) does notoften translate into a high level of composite toughness performance.Due to the combination of thermoplastic and CSR according to embodimentsof the invention, the toughness (G_(1C)) versus viscosity behavior ofthe formulation is closer to that of a CSR toughened material than thatof a thermoplastic toughened material (see FIG. 2).

Example B Comparison of the Variation of CSR Particles to ThermoplasticLoading

Experiments were conducted to quantify the effect of CSR particle (i.e.,MX411) and thermoplastic (i.e., 5003P) loading in the base resin (seeFIG. 2). The viscosity (η) of the material was found to increase withthermoplastic and CSR content. The systems studied displayed viscosityminima onsets which were found to vary with increasing percentage massesof thermoplastic and CSR. The toughness (G_(1C)) behavior of the curedmaterials was found to follow an approximately linear relationship withthe viscosity of the system. Increasing the percentage of thermoplasticand CSR (maintaining the 1 to 0.56 ratio) was shown to give anunexpectedly high increase in the fracture toughness of the neat resinwhen compared to the equivalent thermoplastic loading. The morphology inthe cured materials was shown to follow a similar fashion to thatexpected from samples containing equivalent loadings of analogousthermoplastic (see FIG. 3A, 3B).

Example C Comparison of the Variation of CSR Particles to ThermoplasticRatio

Experiments were conducted to quantify the effect of varying the ratioof CSR particles to thermoplastic (see FIG. 3B). The toughness (G_(1C))behavior of the neat resins was shown to follow a simple linearrelationship as established for other formulations. Additionally, thepresence of CSR and thermoplastic domains in the bulk resin phase wasshown to result in high G_(1C) values for the neat resin. The size ofthe proposed thermoplastic domains in the cured material was found toincrease with CSR content.

Example D Comparison of the Variation of CSR Particles

TABLE D-1 CSR Viscosity at Viscosity at particle 100° C. (P) 130° C. (P)G_(1c) (J m²) T_(g) (° C.) ε (GPa) CSR A 3.1 1.1 233 166 3.57 CSR B 4.21.63 233 165 3.63

Experiments were conducted to compare different CSR particles havingdifferent core chemistries. In this example, “A” ispoly(styrene-butadiene-styrene) or SBS core and “B” is polybutadienecore. There was a negligible viscosity (η) increase with systemsincorporating polybutadiene chemistry (CSR A) relative to systemsincorporating SBS core chemistry (CSR B).

In order to develop a formulation suitable for LRI, prepreg and likeapplications while also resulting in appropriately toughened laminatestructures, the modified resin systems were targeted to remain within athreshold limit of an average viscosity within a temperature range whilemaintaining a high level of toughness (G_(1C)). It was discovered thatformulations according to embodiments of the invention complied with athreshold average viscosity of less than five (5) P with a net weight ofthermoplastic material of less than 30%, more narrowly less than 7%,combined with an amount of CSR particles in a 1 to 0.56 ratio ofthermoplastic to CSR particles, which resultant combined characteristicsrendered the modified resin system suitable for LRI applications. Theviscosity of less than (5) P was discovered to be achievable at atemperature of less than 180° C., more narrowly between 80° C. and 130°C.

According to some embodiments, the thermoplastic material is betweenabout 0.1% and 7% net weight of the modified resin system and the amountCSR particles is between about 0.1% and 10% net weight of the modifiedresin system while maintaining a 1 to 0.56 ratio of thermoplastic to CSRparticles. In one embodiment, the thermoplastic material is about 3.4%net weight of the modified resin system and the amount CSR particles isabout 1.9% net weight of the modified resin system while maintaining a 1to 0.56 ratio of thermoplastic to CSR particles. It was discovered thatthe main contribution to achieving the threshold viscosity was, amongother factors, attributable to the thermoplastic.

Representative formulations according to embodiments of the inventionare illustrated in the following Table 1:

TABLE 1 Base Resin CSR particles Thermoplastic Curing agent Formulation1  7.79% PY-306; 3.89% MX411 in 6.85% 5003P 46.42% 15.58% MY-0510; 3.89%MY-721 MCDEA 15.58% MY-0600 Formulation 2  8.09% PY-306; 2.02% MX411 in3.22% 5003P 48.25% 16.19% MY-0510; 6.04% MY-721 MCDEA 16.19% MY-0600Formulation 3  8.09% PY-306; 2.02% MX416 in 3.22% 5003P 48.25% 16.19%MY-0510; 6.04% MY-721 MCDEA 16.19% MY-0600

Modified Resin Properties

Microcrack Resistance.

Microcrack resistance is the ability of a material to resist formationof small, numerous cracks upon induced stress and strain in the materialwhich instigates localized damage events that eventually weaken andcompromise the composite article. Microcrack resistance is typicallyevaluated using multiple, simulated strain cycles. Samples are withdrawnfor microscopic analysis during the cycle phase and cracks are readilyidentifiable after penetrative staining. During experiments, curedmodified resin samples showed no microcracks after 400 thermal cycles(−53° C. to 90° C.) in one experiment and no microcracks after 2000thermal cycles in another experiment.

Example 2

Modified resin systems and unmodified or partially modified resinsystems were prepared and compared to study crack pinning, ductiletearing and cavitation behavior of the systems expressed in fracturetoughness (K_(1C)) values. The following systems were prepared: (i) amodified resin system having thermoplastic and CSR particles(Formulation 4); (ii) a partially modified resin system havingthermoplastic material (Formulation 5); (iii) a partially modified resinsystem having CSR particles (Formulation 6); and (iv) an unmodifiedresin system (Formulation 7). Examination of the fracture surface ofFormulation 4 illustrated multiple fracture toughness mechanisms atwork. The thermoplastic domains (i.e., 5003P) displayed ductile tearingand crack pinning behaviors while the CSR particle domains (i.e., MX411)exhibited features indicative of a cavitation mechanism (see FIG. 4). Onthe other hand, examination of the fracture surface of the otherFormulations 5, 6, 7 exhibited none or only partial similar damageresistance as that found with respect to Formulation 4. Additionally thecombination of a low concentration of thermoplastic appeared tofacilitate a more homogenous dispersion of CSR particles than inFormulation 6. The following Table 2 summarizes these findings:

TABLE 2 Evidenced toughening Toughening agent mechanism CSR dispersionFormulation 4 Thermoplastic (5003P); Ductile tear; Good CSR particles(MX411) crack pinning: cavitation Formulation 5 Thermoplastic (5003P)Ductile tear; N/A crack pinning Formulation 6 CSR particles (MX411) Tearout Agglomerated Formulation 7 N/A N/A N/A

A numerical evaluation of the fracture toughness (K_(1C)) behaviordemonstrated that Formulations 5, 6, 7 were relatively indistinguishablefrom each other within the experimental parameters as described above ascompared to Formulation 4 (see FIG. 5). The K1C study highlights thesymbiotic relationship of thermoplastic material and CSR particletoughening mechanisms within modified resin systems according toembodiments of the invention. This was supported by an SEM investigationwhich indicated that in the case of the proposed invention, the degreeof ductile failure was observed to be lower than that witnessed informulation 5. Additionally the degree of debonding between thethermoplastic domains in the proposed invention was found to besignificantly less than that witnessed in Formulation 5 (ductile failureand debonding of thermoplastic regions, FIGS. 5 and 6) and it was alsoshown that the CSR domains Formulation 4 exhibited a cavitation-driventoughening mechanism as opposed to the tear out mechanism witnessed inFormulation 6 (FIG. 6).

Morphology Study

Evolution of Morphology as a Function of Temperature.

An investigation was conducted to elucidate the onset point ofmorphology formation in the modified resin system, as embodied by theproposed invention and prepared according to Example 1, during a curecycle. During this investigation, the morphology of the modified resinsystem was determined to generally consist of a phase separation, moreparticularly, an “island-like” morphology, of the thermoplastic and/orCSR particles from the base resin. The “island-like” morphology isgenerally a result of a thermally driven phase separation of thethermoplastic from the base resin into discrete domains ofthermoplastic-rich material identified by a clearly defined border withthe cured or partially cured base resin when the modified resin systemis in a cured or partially cured condition. This morphology was shown toevolve over a sixty (60) minute time interval during ramp-up temperaturefollowed by a constant temperature during a cure cycle. At zero (0)minutes, between 80° C. and 160° C., the modified resin components(thermoplastic, CSR particles and epoxy resin(s)) were shown to be in asubstantially uniform, dispersed phase. Between zero (0) minutes and ten(10) minutes, between 170° C. and 180° C., thermally nucleated “seeds”began to evolve followed by development of these seeds. Between ten (10)minutes and sixty (60) minutes, with the temperature held constant at180° C., thermoplastic domains began to evolve. At about sixty (60)minutes, the morphology of the thermoplastic domains was seen to besubstantially or completely evolved (see FIG. 6). This unique processingfactor, i.e., the controlled and constant morphology evolution developedduring a time period and at a critical temperature (in this case, atabout 180° C.), advantageously avoids flow and filtration issues whichwould otherwise arise from having additive particles of the same size asthe CSR particles in conventional formulations. Through chemicalmodification of the curing agent and the associated control of the resinvitrification point it is expected that the morphology discovered byapplicants will develop at temperatures of less than 180° C.

Effect of Altering Net Percentage of Thermoplastic and CSR Particles.

The morphology of the cured modified resin system (including thedevelopment of thermoplastic domains) was determined to be generallydependent upon the relative concentrations of CSR particles andthermoplastic and, therefore, directly controllable.

Generalized Morphology.

An investigation was conducted to further elucidate the morphology ofthe modified resin system. The investigation was performed by takingimages of the cured resin using a scanned electron microscope (SEM) anda transmission electron microscopy (TEM). The results of the TEM and SEMinvestigations suggest that the thermoplastic domains form via a phaseseparation of thermoplastic from the base resin during the cure of theresin while the CSR particles remain located within the base resin andare not drawn into the thermoplastic domains (see FIGS. 3A, 3B, 4). TheTEM evidence is supported by SEM evidence indicating the presence ofgrowth rings within the thermoplastic morphology (see FIGS. 7A, 8, 9)and also a combined optical microscopy/differential scanning calorimetry(DSC) study demonstrating the onset of morphology growth at the pointwhere the resin begins to vitrify (see FIG. 6).

FIG. 3A illustrates a schematic of the generalized morphology of themodified resin system according to embodiments of the invention asdiscovered by the inventors. As shown in FIGS. 3A-3B (see also FIGS. 4,6-9) addition of thermoplastic within a predetermined range (as well asCSR particles within a predetermined range) to a base resin (having oneor more resins) resulted in a thermally-induced phase separation of thethermoplastic material from the base resin during the cure cycle of themodified resin system. Furthermore, the CSR particles were observed topartially, substantially or completely remain within the base resin andwere not therefore experimentally determined to be incorporated into thethermoplastic material domains.

In addition to being advantageous with respect to processing (seeEvolution of morphology above), the morphology of the modified resinsystem discovered by the inventors is believed to contribute to thecombination of high Compressive After Impact Strength (CSAI), K_(1C)toughness (G_(1C)), and microcrack resistance required for compositearticles exposed to damage caused by environmental conditions and/orevents while simultaneously allowing for a wide processing window duringthe fabrication process. It is anticipated that any thermoplasticexhibiting phase separation morphology, more particularly, an“island-like” morphology, combined with a suitable nanoscale particle(i.e., CSR or hollow particle) would be appropriate for formulatingmodified resin systems according to embodiments of the invention.

Processing Methods Using LRI

FIG. 10 illustrates a representative LRI approach (e.g., Resin Infusionin Flexible Tooling (RIFT)) having a fabric preform thereon. As shown,the system includes a single-sided tool (i.e., mold) 1002 with a fiberpreform 1004 laid thereon. A peel-ply layer 1006 may be applied to asurface of preform 1004. A vacuum bag 1008 having a breather 1010therein seals preform 1004 therein creating a “cavity”, or area in whichpreform 1004 resides. Before preform 1004 is laid on tool 1002, arelease agent or gel coat 1012 may be applied to a surface of tool 1002and/or to a surface of vacuum bag 1008. At one end, the “cavity” isconnected to a resin inlet 1014 via a resin transfer line (not shown).At another end, or at the same end, the “cavity” is connected to avacuum system (not shown) via a vacuum evacuation line 1016. Oncepreform 1004 is positioned within tool 1002 and vacuum is applied, aliquid resin 1018 may be infused into the “cavity” at ambient pressure,a predetermined pressure or a gradient pressure. Liquid resin 1018 maybe infused at ambient temperature, a predetermined temperature or atemperature gradient.

According to embodiments of the invention, modified resin systems (asdescribed previously) may be applied to preforms constructed from one ormore layers of engineered textiles to manufacture composite articlesusing LRI processing techniques and tools (such as that represented inFIG. 10). The engineered textiles may include, but are not limited to,woven fabrics, multi-warp knitted fabrics, non-crimp fabrics,unidirectional fabrics, braided socks and fabrics, narrow fabrics andtapes and fully-fashioned knit fabrics. These fabric materials aretypically formed of fiber glass, carbon fiber, aramid fibers,polyethylene fibers or mixtures thereof. When the preform is subjectedto LRI, LRI-derived laminates are produced.

Representative laminate test samples having the modified resin systemaccording to embodiments of the invention infused therein were preparedaccording to the following general example.

Example 3

Initial lay-ups of non-crimped fiber (NCF) fabric (8 ply layup) wereprepared for RIFT (see FIG. 10) to produce laminate test samples. Inthis embodiment, the fabric was of a carbon material. Laminate testsamples were also prepared using a closed mold RTM press set at 25cm³/minute flow rate and an eight (8) millimeter (mm) inlet. In bothcases, the resin pot was held constant at 100° C. and the tool was heldconstant at 110° C. for infiltration of the resin prior to commencing a2° C. per minute ramp towards 180° C., dwelling for two (2) hours beforeramping down at 2° C. per minute to room temperature. Generally, thetool may be heated to a temperature of between 130° C. and 180° C. at arate of less than 10° C. per minute.

Various tests were performed on the laminate samples in order todetermine compliance with threshold mechanical performance parameters.Key mechanical properties evaluated included storage modulus-derivedglass transition temperature, elastic modulus, Compressive StrengthAfter Impact (CSAI) and open hole compression (OHC) strength (wet anddry).

Laminate Mechanical Properties

Dynamic Mechanical Thermal Analysis (DMTA) was performed to determinethe glass transition temperature (T_(g)) of laminate test samples inaccordance with known methods. Glass transition temperature isindicative of a laminate article to carry mechanical load. Suitableranges are between 130° C. and 210° C. for T_(g) (dry) and between 110°C. and 170° C. for T_(g) (wet). Modified resin systems according toembodiments of the invention were found to have a T_(g) (dry) between140° C. and 190° C. and between 140° C. and 160° C. (wet).

In-plane shear modulus was measured for laminate test samples accordingto known methods. In-plane shear modulus was determined to be between3.5 GPa and 4.5 GPa (dry/RT) and between 3.0 GPa and 4.0 GPa (hot/wet).

Damage Resistance/Tolerance FRP Materials.

Damage resistance is the ability of the composite article to resistdamage after a force event which may cause delamination and weakening ofthe composite article and is a critical parameter for in-servicebehavior in high performance applications. Damage resistance can bemeasured through dent depth analysis or C-scan damage area analysis ofimpacted composite samples. Damage tolerance can be measured by aCompressive Strength After Impact (CSAI) test.

Laminate test samples prepared using modified resin systems according toembodiments of the invention exhibited reduced dent depths when comparedto prior art laminates. In one experiment, laminate test samples werefound to have an average dent depth of between 0.6 mm and 0.8 mmfollowing an impact event. These values represent about a 10% decreasein dent depth when compared to prior art laminates. In anotherexperiment, laminate test samples were found to have CSAI values betweenabout 220 and 270 Mega-Pascals (MPa) in a plain weave textile (see FIG.11) and between about 200 and 225 MPa in a non-crimp fiber textile whichindicate a high tolerance to damage after an impact event. OHC valueswere experimentally determined to be between 280 MPa to 320 MPa (dry)and between 220 MPa and 260 MPa (hot/wet).

The unexpected stable and low average viscosity (i.e., less than 5 P) ofmodified resin systems with a suitable toughness according toembodiments of the invention combined with the high microcrackresistance exhibited by resultant LRI-derived laminate articles rendersit suitable for the manufacture of complex structures in a range ofdifferent industries including the aerospace, transport, electronics,building and leisure industries. Specific to the aerospace industry, themodified resin systems may be used to construct components including,but not limited to, frame and stringer-type components for twin aislederivatives and single aisle replacement programs, fuselage shellcomponents, integrated flight control components for replacementprograms, wing box structures and rotorblade systems for rotorcraft.Additionally, the modified resin systems may be used in the manufactureof composite for complex textile systems.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention is not to be limited to the specific constructionsand arrangements shown and described, since various other modificationsmay occur to those ordinarily skilled in the art.

1. A formulation, comprising: at least one base resin; an amount ofparticles within a predetermined range in a carrier resin; and an amountof thermoplastic material within a predetermined range wherein the baseresin, the particles and the thermoplastic material are combined to forma modified resin system, the modified resin having an average viscositybelow a threshold average viscosity within a predetermined temperaturerange.
 2. The formulation of claim 1, further comprising, ananiline-based amine compound curing agent.
 3. The formulation of claim 1wherein the base resin is one of epoxy, bismaleimide, cyanate ester or acombination thereof.
 4. The formulation of claim 3 wherein the baseresin comprises a combination of epoxies including at least one di- andtri-epoxy and at least one tetra-epoxy.
 5. The formulation of claim 1wherein the particles are one of functionalized core-shell rubberparticles or hollow particles.
 6. The formulation of claim 1 wherein theparticles are one of non-functionalized core-shell rubber particles orhollow particles.
 7. The formulation of claim 5 wherein thefunctionalized core-shell rubber particles comprise a core materialwhich is one of polybutadiene-styrene, polybutadiene or a combinationthereof, and a shell material which is one of silica, polymerizedmonomers of acrylic acid derivatives containing the acryl groupincluding acrylic and poly(methyl methacrylate) or a combinationthereof.
 8. The formulation of claim 6 wherein the non-functionalizedcore-shell rubber particles comprise a core material which is one ofpolybutadiene-styrene, polybutadiene or a combination thereof, and ashell material which is one of silica, polymerized monomers of acrylicacid derivatives containing the acryl group including acrylic andpoly(methyl methacrylate) or a combination thereof.
 9. The formulationof claim 1 wherein, in a cured condition, the particles aresubstantially uniformly dispersed throughout the modified resin system.10. The formulation of claim 1 wherein the thermoplastic materialcomprises one of phenoxy-based polymers, poly(ether sulfone) polymers,poly(ether ether sulfones), poly(methyl methacrylate) polymers,carboxylterminated butadiene acrylonitrile polymers, copolymers thereofor combinations thereof.
 11. The formulation of claim 1 wherein theamount of thermoplastic material is below approximately 30% net weightof the modified resin system.
 12. The formulation of claim 1 wherein, ina cured condition, at least the thermoplastic material is phaseseparated from the base resin.
 13. The formulation of claim 1 whereinthe threshold average viscosity is less than 5 Poise at a temperature ofless than 180° C.
 14. A composite article, comprising: a structurehaving a predetermined shape, the structure having a plurality of layersof a fiber-based fabric, the structure having a targeted compositetoughness within a predetermined range, wherein the toughness is atleast partially imparted by a modified resin system during a process,the modified resin system including: (i) at least one base resin; (ii)an amount of particles within a predetermined range in a carrier resin;and (iii) an amount of thermoplastic material within a predeterminedrange wherein the base resin, the particles and the thermoplasticmaterial are combined to form the modified resin system, the modifiedresin having a average viscosity below a threshold average viscositywithin a predetermined temperature range.
 15. The composite article ofclaim 14 wherein the structure exhibits a high level of microcrackresistance.
 16. The composite article of claim 14 wherein the process isa liquid resin infusion manufacturing process, a prepreg manufacturingprocess or a resin film infusion process.
 17. A formulation, comprising:a base resin comprising at least one epoxy; a curing agent; an amount ofthermoplastic material; and an amount of core-shell particles whereinthe base resin, the curing agent, the thermoplastic material and theparticles are combined to form the modified resin system, the modifiedresin having an amount of thermoplastic material of less than 30% netweight of the total weight of the modified resin system.
 18. Theformulation of claim 17 wherein, in a cured or vitrified condition, thethermoplastic material separates into aggregate domains from the baseresin, each aggregate domain having an island-like morphology, themorphology evolving (i) during the later stages of a ramp to dwelltemperature or (ii) after a ramp to dwell has been completed during thecure cycle.
 19. A manufacturing process, comprising: preparing apreform; laying the preform within a mold; heating the mold to apredetermined temperature; and injecting a resin wherein the resin is amodified resin, the modified resin system comprising a combination of:(i) at least one base resin; (ii) a curing agent; (iii) an amount ofparticles within a predetermined range in a carrier resin; and (iv) anamount of thermoplastic material within a predetermined range whereinthe amount of thermoplastic material of the modified resin is less than30% net weight of the total weight of the modified resin system.
 20. Themanufacturing process of claim 19 wherein the predetermined temperatureof the mold is less than 180° C.
 21. The manufacturing process of claim19, further comprising, ramping a temperature of the mold to 180° C. ata rate of up to 10° C. per minute.
 22. The manufacturing process ofclaim 19 wherein, when the mold reaches 180° C., the temperature is heldfor between 30 minutes and 150 minutes.